When monochromatic light is directed on a molecule, the light can be either absorbed or scattered. When the light strikes the molecule, the electrons in the molecule will absorb the energy of the light wave and change their energy state. Once this occurs, either the energy is retained by the matter and the light is absorbed, or the electron returns to a lower energy state emitting a photon of light.
If the photon energy is absorbed, the energy from the photon is transformed to other forms of energy, such as heat. In absorption, the frequency of the incoming light wave is at or near the energy levels of the electrons in the matter. However, if the photon is immediately re-emitted, the photon is effectively reflected, or scattered.
When photons are scattered from an atom or molecule, most of the scattered photons have the same kinetic energy (frequency) and wavelength as the incident photons. This type of scattering is elastic scattering and has been named Rayleigh scattering. However, a small fraction of the scattered photons, approximately 1 in 10 million, is scattered by an excitation, wherein the scattered photons have an energy (frequency) that is different from, and usually lower than, that of the incident photons. This type of scattering is inelastic scattering and has been named Raman scattering.
Rayleigh scattered photons generally do not provide any useful information for molecular characterization. Raman scattered photons, on the other hand, are able to provide information about vibrational, rotational and other low frequency transitions in molecules. Detection of the Raman scattering by Raman spectroscopy can therefore be used to study solid, liquid and gaseous samples.
Raman scattering intensity is orders of magnitude weaker than the Rayleigh signal. For example, for gases, the Raman scattering signal is weaker than the Rayleigh signal by a factor of 103. For solids, this difference can be more than 106. The Raman scattering signal can therefore be extremely difficult to detect. It is therefore desirable to enhance the Raman scattering signal to enable trace element detection at lower concentrations, such as parts per billion (ppb) levels.